1. Field of the Invention
The present invention relates generally to a method for manufacturing microthrough-hole and a circuit board structure with microthrough-hole, and more specifically to a method for manufacturing microthrough-hole in a circuit board by using the pitch of the buried metal circuit as the diameter of the microthrough-hole, and employing the aspect that a laser beam easily penetrates insulation material but difficultly through metal material so as to effectively shrink the microthrough-hole, reduce its occupied area and achieve the demand of high circuit density.
2. The Prior Arts
In general, printed circuit boards (PCBs) are necessary for all electronic products to mount various electronic elements.
The PCBs are commonly categorized as single board, double boards and multiple boards. Recently, electronic gadgets have become much smaller, lighter, thinner, faster, and required more excellent performance, higher circuit density and lower cost. In particular, the present package technologies have a trend of high pin count, fine pitch and packed integration. As a result, it is very changing for the PCBs to implement the features of high density circuit layout, miniaturized holes, composite multiple layers and ultra thin plate. The processes of building up multiple layers and high density interconnection (HDI) are two widely used schemes for the multiple layer PCBs.
Please refer to FIGS. 1A-1D showing the interlayer connection structure in the prior arts. As shown in FIG. 1A, the first circuit 7a is formed on the first stack plate 1a. Next, the insulation layer 3a is formed on the first stack plate 1a and the first circuit 7a, and the second circuit 11a is formed on the insulation layer 3a (FIG. 1B). The pattern of copper window 9a is formed by etching the second circuit 11a through a process of manufacturing copper window (FIG. 1C). A laser beam is employed to drill through the insulation layer 3a at the copper window 9a to form the through-hole 14a on the first circuit 7a as the stop pad (FIG. 1D).
Traditional microvia is primarily manufactured by the laser drilling process such as conformal mask drilling, enlarge window drilling and copper direct drilling. Further refer to FIG. 1D showing the process of opening copper window in the prior arts. The insulation layer 3a, the second circuit 11a and the first circuit 7a are sequentially formed on the first stack plate 1a. Then, part of the second circuit 11a is etched to form the opening (copper window 9a), and the opening is processed by the laser 16a to form the through-hole 14a on the insulation layer 3a. 
However, the diameter of the through-hole manufactured by the process of copper window is limited by the size of the laser beam passing through the mask, and particularly the processes of photoresist development and photolithography. As a result, the size of the window for the through-hole can not be further reduced and the circuit density of the circuit board is hard to increase.
Specifically, since it is difficult for the copper foil to absorb the laser, the process of photoresist development needs the help of the photolithography to form the opening in the second circuit 11a. Based on the current technology, the window formed on the patterned photoresist layer has a minimum size larger than 50 μm. Thus, the opening formed by etching the second circuit 11a is surely larger than 50 μm.
If the copper window is further widened in the copper window process, the usage of the available circuit area of the second circuit 11a is reduced. While the process of direct laser may omit the process of forming the copper window, the absorption of the laser beam needs to increase and the size of the opening is still limited by the beam size through the mask.
In addition, the process of desmear performed after laser drilling greatly widens the opening size such that the pitch in the final product is only 140 μm.
Therefore, it is greatly needed for the method for manufacturing microthrough-hole, in which the first circuit and the copper window are first formed, only the metal material is treated to improve stability of the process, the process of patterned dry film electroplating is used to form the metal circuit having line width and pitch less than 50 μm, the metal circuit is buried in the insulation layer, only the insulation layer is penetrated during the laser drilling, and the size of the microthrough-hole is the same as the size of the copper window so as to achieve the object of high circuit density, thereby overcoming the above problems in the prior arts.